Hadrontherapy was proposed by Robert R. Wilson in 1946 and is based on the energy deposition characteristics of charged ions in matter. It is aimed in particular at improving the treatment of cancers by virtue of excellent ballistic precision and optimal biological effectiveness, close to 1 in healthy tissues and of the order of 2 to 3 in the tumoral volume (the biological effectiveness of a radiation is defined as the ratio of the doses with X rays and with the relevant radiation so as to obtain one and the same biological effect, typically a survival rate for a cellular population of 10%).
Indeed, in contradistinction to conventional radiations, such as photons (X or gamma) or electrons, for which the profile of the dose delivered to the tissues decreases progressively with depth traversed, that of ions allows a high dose deposition at the end of the track (dubbed a Bragg peak) whereas the dose deposited upstream (corresponding to the so-called plateau region) is much lower. The depth-wise position of the Bragg peak, which is controlled by the incident energy of the beam of charged hadrons, may be modified, thus making it possible to deposit the maximum energy within a circumscribed target volume, in particular a tumor, while sparing the healthy tissues upstream and downstream.
In a manner known per se the ionized hadrons are accelerated by a cyclotron or a synchrotron and the energy of the particle on exiting the accelerator determines the depth of penetration and the position of the maximum biological effectiveness of the irradiation.
By virtue of these properties, allied with weak lateral spreading, the dose deposited in the tissues by charged hadrons may be confined with markedly greater precision than in conventional radiotherapy.
Among the hadrons, light hadrons such as protons or carbon ions are preferably chosen. Carbon ions are particularly advantageous since they exhibit substantially better ballistics than those of protons (less lateral dispersion) and optimal biological effectiveness.
The interaction of the hadrons with the tissues may, when there is inelastic collision between the projectile and target nuclei, create hadron fragmentation phenomena which produce, in particular, unstable nuclei, gamma radiations and neutrons.
For ease of speech, the term “gamma” may be employed to denote a gamma ray.
According to a known method, the emission of positions (for example emitted by 11C nuclei in the case of a beam of 12C6+ ions), is used to measure and/or visualize the dose distribution arising from the interaction between the hadrons and the target. It is in particular possible to use Position Emission Tomography (PET) techniques to this end. However, this technique exhibits the drawback of being an a posteriori measurement which does not make it possible to follow the evolution of the doses received during treatment.
Furthermore, it has been noted that the distribution of the position-emitting particles in the target cannot always be reliably correlated with the dose distribution in the target.
In order to alleviate these drawbacks, a technique for measuring prompt gamma rays has been proposed by Min et al. for controlling the track of the protons during irradiation (Chul-Hee Min and Chan Hyeong Kim, Min-Young Youn, Jong-Won Kim “Prompt gamma measurements for locating the dose falloff region in the proton therapy”, Applied Physics Letters 89, 183517 (2006)).
According to this technique, a prompt gamma ray scanner is implemented, which comprises three layers of screens surrounding a detector to form a barrier to the neutrons produced by the nuclear fragmentation. A first layer of paraffin wax moderates the high-energy neutrons, and then a layer of B4C powder makes it possible to capture a good part of the neutrons and the detector is finally stationed within a layer of lead which blocks the undesired gamma radiations. The total length of the protection layers of such a scanner is about 70 cm which results in a bulky apparatus that is awkward to handle and rather ineffective.